The present invention generally relates to air cycle Environmental Control Systems (ECSs). More specifically, the invention relates to an improved two spool ECS and improved method of conditioning water vapor compressed air by utilizing two spool subsystems in series with one another while increasing efficiency and flexibility.
ECSs are used to provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. In the past, an air cycle ECS has typically operated on a flow of bleed air taken from an intermediate or high pressure stage within a jet engine having multi-compression stages. The bleed air has usually been pre-cooled within a primary heat exchanger with heat being dumped to RAM air and then flowed to a compressor. After compression, the air has been routed through a series of heat exchangers and condensers. Then, the air has typically been expanded by a turbine which is mechanically engaged to the compressor. Finally, the air can be sent to the cabin.
Past air cycle ECS designs have included 2, 3 and 4 wheel bootstrap, high pressure water separation cycles. The general distinction among the three designs relates to the number of so-called wheels which are mechanically engaged to one another. All three of the bootstrap designs typically utilize a reheater and a condenser heat exchanger to respectively pre-cool the bleed air and then condense the water vapor in it. After condensation, the condensed water is removed by a water extractor. The resulting dehumidified air flows to the reheater where the remaining water droplets are evaporated, leaving the residual moisture in the vapor phase. The dry air then flows to a turbine for expansion and consequent cooling. The expansion will typically cool the air to below freezing temperature and thus the vapor particles form ice nuclei and crystallize into snow, which are swept downstream. The expanded air from the turbine can then be used to cool and condense water in the condenser heat exchanger.
For the 2 and 3 wheel system, the expanded air which has been warmed in the condenser can then be directly supplied to a cabin. However, the differentiating feature between those two systems is that the 2 wheel typically has the turbine engaged to a compressor, while the 3 wheel has the turbine engaged to the compressor as well as a fan which pulls RAM air through the system. In the 4 wheel design, shown for example in U.S. Pat. No. 5,086,622, the expanded air which has been warmed in the condenser is then further expanded by another turbine for eventual supply to the cabin. That design has the two turbines engaged to the compressor and fan, i.e., 4 wheels. Also, the design in U.S. Pat. No. 5,086,622 does not flow the dehumidified air through a reheater prior to entering the first turbine. That presents a disadvantage since the residual condensed water droplets in the first turbine inlet stream impinge on the cold turbine blades and outlet walls and freeze out if the metal temperatures are much below freezing. Ice then quickly accumulates and must be rapidly melted to avoid clogging the cycle.
A common disadvantage to the 3 and 4 wheel bootstrap systems is that it creates an "off-design" limitation. In particular, the fan is forced to operate at the same speed as the compressor and turbine(s), even though the fan typically finds optimal performance at a speed lower than the compressor and turbine(s). Thus, a compromise is made in design optimization, which has usually been balanced in favor of the compressor and turbine(s). The 2 wheel system shown in U.S. Pat. No. 4,198,830 partially ameliorates the "off-design" limitation by incorporating a 2 spool design. In other words, the fan is engaged to a turbine by one spool and another turbine is engaged to the compressor by another spool. The spools operate independently of one another by having bleed air separately routed to each spool. Accordingly, the spools can be said to be operating in "parallel" to one another. Thereby, the fan can operate at a speed independent of that of the compressor and its related turbine, which has often been about one-fourth the speed of the compressor/turbine.
Yet, having spools parallel to one another in the 2 wheel bootstrap system creates energy inefficiencies. With the parallel design, the fan and its related turbine operate off the bleed air before it is compressed and conditioned. In contrast, the compressor and its related turbine(s) operate off the bleed air upon being compressed and conditioned. Thus, during auxiliary power unit operation, while a majority of the bleed air (perhaps about 87%) is subject to being conditioned, it is not all of the bleed air. The consequence is that, among other things, the cooling capacity is reduced. Also, if only a small portion of the bleed air (perhaps about 13%) is going to turn the fan, there is less fan power as compared to a situation where all of the bleed air is used. Less fan power translates into requiring larger RAM air heat exchangers. Another energy inefficiency in the prior 2 wheel system is that the heat of condensation and sensible cooling is lost to the supply air. That is due to the fact that the supply air typically comes directly from the condenser, with no downstream means of recovery. Furthermore, the past 2 wheel system has typically provided no means for utilizing the spool containing the fan as an alternative conditioning spool in the event of a failure by the other spool.
As can be seen, there is a need for an improved two spool ECS and method of conditioning high pressure water vapor bearing air which effectively increases cooling capacity by decreasing the required size of heat exchangers. There is an additional need for such a system and method which increases efficiency by recovering the heat of condensation and sensible cooling that might otherwise be lost to the supply air, for example. A further need is a two spool ECS and method which provides flexibility in use, including the ability to still provide conditioned air when one of the two spools is non-operational.